1. Field of the Invention
This invention relates to a supercharging pressure control system for supercharged internal combustion engines.
2. Prior Art
Conventionally, there has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 58-170825, a supercharging pressure control system for a supercharged internal combustion engine which has a supercharger arranged in the intake passage of the engine, a bypass passage bypassing the supercharger, a supercharging pressure control valve for opening and closing the bypass passage, driving means for driving the supercharing pressure control valve, and a control device for controlling the driving means, wherein a set value of supercharging pressure determined according to the rotational speed of the engine and a signal indicative of a degree of acceleration demanded (i.e. opening of a throttle valve) is compared with an actual value of supercharging pressure detected in the intake passage, and the opening of the supercharging pressure control valve is controlled in response to a signal generated based on results of the comparison. That is, according to this prior art system, the opening of the supercharing pressure control valve is feedback-controlled over the whole region of supercharging pressure to make the supercharging pressure equal to the set valve thereof. However, such a feedback control of supercharging pressure over the whole region thereof is slow in control speed in a transient state of supercharging pressure controlled.
To eliminate this inconvenience, a control method may be used in a supercharging pressure control system for a supercharged internal combustion engine, which method comprises feedback-controlling the supercharging pressure (pressure P2 upstream of the throttle valve or pressure PB downstream of same) in a high pressure region alone, and open loop-controlling same in regions outside the high pressure region. More specifically, according to the method, when the engine rotational speed (NE) or the throttle valve opening (.theta.TH) has reached a corresponding set value, the control mode is changed from an open loop control mode in which the opening of the supercharging pressure control valve is open loop-controlled to a desired value (map value) dependent on operating conditions of the engine, i.e. the engine rotational speed NE and the throttle valve opening .theta.TH, to a feedback control mode in which the opening of the supercharging pressure control valve is feedback-controlled so as to make the supercharging pressure equal to a desired value thereof, whereas when the engine rotational speed NE or the throttle valve opening .theta.TH becomes lower or smaller than the corresponding set value, the control mode is changed from the feedback control mode to the open-loop control mode.
Further, there has also been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 62-153523, a supercharging pressure control system for a supercharged internal combustion engine, which is adapted to perform the feedback control when the vehicle on which the engine is installed is cruising, and when the vehicle is quickly accelerated, the feedback control is inhibited to perform the open-loop control by the use of a learned value of the opening of the supercharging pressure control valve obtained during the feedback control, to effect smooth transition from the open-loop control to the feedback control.
However, according to the former prior art in which the supercharging pressure is feedback-controlled in the high pressure region alone, and open-loop controlled in the other regions, if the intake air is at normal temperature, there is almost no variation in a value of the supercharging pressure assumed before transition from the open-loop control mode to the feedback control mode, which is carried out when the engine rotational speed NE or the throttle valve opening .theta.TH has reached the corresponding set value, and at the same time this value is approximately equal to the desired value of supercharging pressure used in the feedback control. Therefore, this change in the control mode causes no shocks, such as unexpected disturbing vibrations or noise. However, if the temperature of intake air varies from normal temperature due to a change in the ambient temperature or operating conditions of the engine, such as the speed of the vehicle, the value of the supercharging pressure assumed immediately before the transition varies with this variation in the intake air. For example, when the temperature of intake air is much lower than normal temperature, the supercharging pressure value assumed immediately before the transition becomes much lower than the desired value of supercharging pressure, and the supercharging pressure sharply rises to the desired value of supercharging pressure in the transition from the open-loop control to the feedback control, which results in occurrence of shocks, such as unexpected disturbing vibrations and noise. Even if the desired value of supercharging pressure is corrected based on a signal from an intake air temperature sensor, occurrence of the shocks cannot be avoided due to delayed response of the sensor. On the other hand, when The intake air temperature is much higher than normal temperature, the supercharging pressure value assumed immediately before the transition becomes much higher than the desired value of supercharging pressure, so that the supercharging pressure drastically drops to the desired value in the transition, which similarly causes the above-mentioned shocks.
The same shocks will occur in transition from the feedback control to the open-loop control. More specifically, if the intake air is at normal temperature and the engine is under the same operating conditions, upon transition from the feedback control to the open-loop control after the engine rotational speed NE or the throttle valve opening .theta.TH becomes lower or smaller than the corresponding set value, there is almost no variation in a value of the supercharging pressure assumed immediately after the transition, and at the same time the value of the supercharging pressure is approximately equal to the desired value of supercharging pressure. Therefore, no shocks such as unexpected vibrations and noise will occur. However, if the intake air temperature becomes much lower than normal temperature due to a change in the operating conditions of the engine, the supercharging pressure value to be assumed immediately after the transition also becomes much lower than the desired value of supercharging pressure. Therefore, in the transition from the feedback control to the open-loop control, the supercharging pressure drastically drops from the desired value of supercharging pressure, producing shocks, such as unexpected vibrations and noise. When the intake air temperature is much higher than normal temperature as well, the same shocks are produced, since the supercharging pressure value to be assumed after the transition is much higher than the desired value of supercharging pressure, and hence the supercharging pressure sharply rises from the desired value of supercharging pressure.
According to the latter prior art (Japanese Provisional Patent Publication (Kokai) No. 62-153523), in which the feedback control is inhibited, i.e. the open-loop control is carried out by the use of the learned value of opening of the supercharging pressure control valve obtained during the feedback control, when the engine is quickly accelerated, if there is a large difference in a value of the intake air temperature at which the learned value of opening of the supercharging pressure control valve is calculated and stored, and a value of the intake air temperature at which the control mode is changed from the open-loop control mode to the feedback control mode, the above-mentioned shocks, such as the unexpected disturbing vibrations and noise, are produced during the transition from the open-loop control to the feedback control.